JP5021344B2 - Method for concentrating light calcium carbonate - Google Patents

Description

  The present invention relates to a needle-like or columnar light carbonic acid produced by a causticizing step of a pulp manufacturing process by a sulfate method or a soda method, wherein quick lime is dehydrated with water or a weak liquid and then causticized with a green liquor. The present invention relates to a method of highly concentrating calcium slurry so that it can be used for pigment applications.

Since light calcium carbonate is excellent in whiteness and opacity, its use amount is increasing year by year as a coating pigment for papermaking. In general, the calcium carbonate slurry preferably has a solid content of 70% by weight or more in order to reduce the drying load of the coated paper. Light calcium carbonate is a causticizing process of the pulp manufacturing process by the method of producing carbon dioxide by blowing carbon dioxide into the slaked lime slurry obtained by soaking quick lime and reacting it with the sulfate method or soda method. There is a method (causticizing method) in which water is weakened with water or a weak liquid and then causticized with a green liquid. Commercially available light calcium carbonate for coating is produced by the carbon dioxide method, and the particle shape is usually needle-like or spindle-like. In particular, acicular light calcium carbonate is excellent in opacity and white paper glossiness, so it can be easily replaced with kaolin and is suitable for improving the quality of coated paper. About the acicular light calcium carbonate (arre stone-type PCC) slurry (solid content concentration: 16 to 18% by weight) for coating obtained by the carbon dioxide method, a pressure filtration device that operates under a pressure of 5 to 10 MPa, In particular, a method of highly concentrating to 71 to 72% by weight using a tube pressure filter has been proposed (Patent Document 1).
JP-A-9-202617

  However, when the acicular or columnar light calcium carbonate having a solid content of 20% by weight obtained in the causticizing process is treated at 10 MPa using a tube pressure filter, it can be concentrated to a solid content of 70% by weight or more. It was found that it was difficult to peel off from the filter cloth. Usually, this peelability can be improved by increasing the cake thickness, so it was considered that the cake was thickened by repeating the slurry introduction and draining treatment, but this method clogged the filter cloth of the tube pressure filter. It becomes easy and there is a difficulty in continuous operability. Therefore, the present invention is a method for highly concentrating a needle-like or columnar light calcium carbonate by a causticizing method to a solid content concentration of 70% by weight or more without any problem in cake peelability, using a dewatering device instead of a tube pressure filter. The purpose is to provide.

  In order to solve the above-mentioned problems, the present inventors, as a tube press filter, have conducted intensive research on the dewatering conditions in a high-pressure filter press that can be dewatered at ultra-high pressure and can continuously supply slurry to the filter chamber. The present inventors have found that the cake solid content can be improved to a concentration that can be used for pigment applications by optimizing the filter chamber thickness and processing pressure of the filter press apparatus.

  That is, acicular or columnar light calcium carbonate slurry having an average particle diameter of 2 μm or more obtained in the causticizing step is 8 to 10 MPa, preferably 10 MPa, for 20 to 60 minutes, preferably 20 to 20 MPa in a high pressure filter press. The cake solid content concentration is set to 70% by weight or more, preferably 72% by weight or more by pressure concentration for 30 minutes, and the cake thickness is set to 70 mm or more, preferably 80 mm after completion of dehydration. It is a high concentration method for light calcium carbonate.

  According to the present invention, it is possible to concentrate needle-like or columnar light calcium carbonate obtained in a causticizing process, which could not be highly concentrated in the prior art, to 70% by weight that can be used for pigment applications.

Hereinafter, the method of the present invention will be specifically described. The light calcium carbonate of this invention uses what was manufactured at the causticizing process of the pulp manufacturing process by a sulfate method or a soda method.
As a light calcium carbonate production method, (1) a reaction between carbon dioxide-containing gas obtained from a lime baking apparatus and others and lime milk, (2) a reaction between ammonium carbonate and calcium chloride in an ammonia soda method, ( 3) Reaction of lime milk and sodium carbonate, which produces sodium hydroxide by causticization of sodium carbonate, is known. Among these methods, in (2) and (3), the production method for obtaining the main product has been changed to a new method, or because calcium carbonate is a byproduct, the content of impurities is high. Has not been studied much.

On the other hand, (1) has a relatively simple reaction system (water, slaked lime, carbon dioxide), and extensive research has been conducted on methods for producing calcium carbonate suitable for various purposes. Some products are also seen. However, the direct purchase from the manufacturer has the disadvantage that the transportation cost is high and the total cost is high. Further, in the on-site carbon dioxide method, if kiln exhaust gas is used, high-quality calcium carbonate can be produced at a low cost, but there is a problem that the capital investment is enormous.
One possible method is to use calcium carbonate, which is a by-product when producing white liquor in the causticizing process for recovering and regenerating cooking chemicals, in the pulp manufacturing process using the sulfate method or soda method. is there. This method has the advantage that existing facilities can be used and the capital investment is minimized. In particular, when acicular or columnar light calcium carbonate having an aragonite crystal structure is used for pigments, a coated paper excellent in gloss development and opacity can be produced.

  In the pulp manufacturing process by the sulfate method or the soda method, cooking is performed under high temperature and high pressure using a chemical solution in which sodium hydroxide and sodium sulfide are mixed in order to isolate wood fiber. The fiber is separated and purified as a solid layer to become pulp, and the elution components other than the chemical solution and the fiber from the wood are recovered as pulp waste liquid (black liquor) and concentrated to a combustible concentration in the recovery boiler. Further, sodium sulfate is added to replenish sodium and sulfur lost in the series of processes, and then burned in a recovery boiler. At that time, the organic substance in the black liquor is recovered as a heat source, and the inorganic substance is mainly recovered as sodium carbonate and sodium sulfide. These inorganic substances are called smelt and are taken out from the recovery boiler in a molten state. The smelt taken out from the recovery boiler is dissolved in water or a weak liquid (a liquid containing a small amount of white liquor component obtained after washing calcium carbonate with water) to become a green liquid.

  The causticizing process is a process for changing sodium carbonate in green liquor to sodium hydroxide, a cooking chemical. Mixing the slaked lime and green liquor with a hydration reaction (1) to convert quick lime to slaked lime. It consists of a causticizing reaction (2) that produces sodium and calcium carbonate. The liquid obtained by the causticizing reaction is called white liquor, separated from calcium carbonate, clarified, and sent to the cooking process. In the present invention, calcium carbonate that has been separated and recovered and thoroughly washed is used.

CaO + H ₂ O → Ca (OH) ₂ (1): Saturation reaction
Ca (OH) ₂ + Na ₂ CO ₃ → CaCO ₃ + 2NaOH (2): Causticizing reaction This calcium carbonate is a by-product in producing white liquor, which is the main product. Compared to the light calcium carbonate obtained by the reaction method, there is an advantage that the existing equipment can be used and the capital investment is minimized. Also, by extracting calcium carbonate out of the system from the calcium (quick lime, slaked lime, calcium carbonate) circulation cycle of the causticization process in the conventional closed system, purification in the system and high purity of the circulating lime are achieved. 1) The reactivity improvement of (2), the improvement of the clarity of white liquor, and the reduction of waste can be expected.

  Furthermore, the acicular or columnar light calcium carbonate defined in the present invention is produced according to the following production method. That is, (1) Using quick lime introduced from outside the causticizing process, (2) The molar ratio of quick lime and water at the time of slaked lime generation is quick lime: water = 1: 1 to 1.5, and the weight of slaked lime White liquor is added to the slaked lime containing 0.05 to 8% by weight of calcium carbonate as a reference so that the slaked lime concentration is 10 to 60% by weight, and the mixture is hydrated with stirring or mixing to make lime A first stage step for producing milk and / or lime mud, and then, the lime milk and / or lime mud are produced in the causticizing step and a predetermined amount of green liquor necessary for producing white liquor is added to the lime. It is manufactured by sequentially adding to milk and / or lime mud at an addition rate within a specific range and performing a causticizing reaction at a reaction temperature of 25 to 75 ° C.

The average particle size of the acicular or columnar light calcium carbonate obtained in the causticizing step is preferably 2 μm or more as measured by a laser transmission type particle size distribution analyzer (Mastersizer 2000, manufactured by Malvern). When the thickness is less than 2 μm, the liquid removal property is remarkably lowered, and the cake solids concentration cannot be increased to 70% by weight or more even if the filter chamber thickness is 70 mm or more and the pressure treatment is performed at 10 MPa.
In the case of 2 μm or more, if the cake thickness is less than 70 mm, the cake solid content concentration cannot be made 70% by weight or more.
If the applied pressure is less than 8 MPa, the cake solids concentration cannot be made 70% by weight or more.
When the applied pressure exceeds 10 MPa, the cake solid content concentration can be increased to 70% by weight or more. However, it is necessary to reinforce the member so that the high-pressure filter press part can withstand ultra-high pressure. There is a problem such as securing of space due to enlargement.

  The high-pressure filter press is superior in dewatering performance because the slurry press-fitting pressure during filtration is set higher than that of conventional dewatering machines such as filter presses, vacuum dehydrators, and belt presses. The high-pressure filter press has two units: a high-pressure driving pump that generates slurry pressure and a high-pressure filter press main body that performs dehydration. As auxiliary devices, there are a device for peeling the dehydrated cake from the filter cloth of the filter press and a cleaning device for preventing clogging of the filter cloth surface. FIG. 1 shows a high-pressure filter press used in the present invention. FIG. 2 shows that this apparatus comprises four steps: (1) preparation for dehydration filtration, (2) dehydration filtration, (3) residual liquid blowing, and (4) cake peeling.

The cake thickness after completion of dehydration can be arbitrarily controlled by the thickness of the filter chamber of the high-pressure filter press. In FIG. 2, “after dehydration” means after completion of dehydration filtration.
The average particle size can be controlled by the stirring force during the reaction, the temperature of the green liquor and the time for adding it. In particular, the stirring force has a large influence on the particle size, and the particle size increases as the stirring force is decreased.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is of course not limited thereto. In the examples, “parts” and “%” represent “parts by weight” and “% by weight”, respectively, unless otherwise specified.

The acicular light calcium carbonate slurry with a solid content concentration of 30% produced in the causticizing process of Nippon Paper Industries Co., Ltd. A Factory was used. Moreover, the high pressure filter press apparatus of Nippon Steel Plant Design Co., Ltd. was used as a liquid removal apparatus. Light calcium carbonate slurries with different average particle diameters were processed by changing the pressure applied during the concentration of the filter press apparatus and the filter chamber thickness, and the solid content% of the resulting cake was measured.
[Example 1]
An acicular light calcium carbonate slurry having an average particle size of 2.5 μm was treated with a cake thickness of 85 mm, a pressing force of 10 Mpa during concentration, and a pressure concentration time of 25 minutes.
[Example 2]
A needle-like light calcium carbonate slurry having an average particle size of 5.7 μm was treated with a cake thickness of 70 mm, a pressure of 8 Mpa during concentration, and a pressure concentration time of 25 minutes.
[Comparative Example 1]
An acicular light calcium carbonate slurry having an average particle diameter of 1.1 μm was treated with a cake thickness of 70 mm, a pressure of 10 MPa during concentration, and a pressure concentration time of 45 minutes.
[Comparative Example 2]
A needle-like light calcium carbonate slurry having an average particle size of 5.7 μm was treated with a cake thickness of 40 mm, a pressure of 8 MPa during concentration, and a pressure concentration time of 25 minutes.
[Comparative Example 3]
A needle-like light calcium carbonate slurry having an average particle size of 5.7 μm was treated with a cake thickness of 70 mm, a pressure of 6 MPa during concentration, and a pressure concentration time of 25 minutes.

  From Table 1, in a method of highly concentrating acicular light calcium carbonate slurry obtained by a causticizing method having an average particle size of 2 μm or more with a high-pressure filter press apparatus, the cake thickness after dehydration is 70 mm or more and 8 to 10 MPa. It can be seen that the cake solids concentration can be increased to 70% or more by concentrating under pressure.

It is an example of the high pressure filter press used by this invention. It is a figure which shows four processes of the high pressure filter press of FIG.

Claims (1)

  A needle with an average particle size of 2 μm or more produced by causticizing the green liquor after the quick lime is dehydrated with water or weak liquor in the causticizing step of the pulp manufacturing process by the sulfate method or soda method In a method of highly concentrating a cylindrical or columnar light calcium carbonate slurry with a high-pressure filter press apparatus, the cake solid content is set to 70% by weight or more by pressurizing and concentrating at 8 to 10 MPa for 20 to 60 minutes. A method for concentrating light calcium carbonate, wherein the thickness is 70 mm or more.

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